A wide variety of electrically powered heating devices are in existence to provide a wide variety of functions. For example, temperature calibration devices, known as dry well calibrators, are commonly used in industry to calibrate precision temperature probes.
Conventional dry well calibrators use thermoelectric heating/cooling modules generally containing Peltier cells to heat or cool the calibration probes to temperatures that can be set by a user. Electrical power having one polarity is applied between the first and second substrates of the Peltier cells to cause the temperature of the first substrate to rise relative to the temperature of the second substrate, thereby heating the temperature probe being calibrated. Electrical power having the opposite polarity causes the temperature of the first substrate to fall relative to the temperature of the second substrate, thereby cooling the temperature probe being calibrated.
Peltier cells used in dry well calibrators are usually stacked on top of each other to provide heating and cooling over a range of temperatures that is wider than the temperature differential of each cell. The total temperature differential of a heating/cooling module is substantially equal to the sum of the temperature differentials that can be developed across all of the stacked Peltier cells. The temperature differential that can be developed between the substrates of each Peltier cell is limited to a specified maximum temperature. Therefore, the limiting factor in the operating range of a dry well calibrator is the maximum specified temperature differential of the Peltier cells used in the dry well calibrator. This limiting effect on the operating range of dry well calibrators is exacerbated by the unequal heating of the Peltier cells. Specifically, the temperature differential of Peltier cells in the outside of a stack tend to be greater than the temperature differential of cells that are located toward the inside of the stack. To limit the temperature differential of the cells at the outside of the stack to the specified maximum temperature differential, the other cells in the stack are usually well below the maximum specified temperature differential. Therefore, the maximum operating range of dry well calibrators is typically much smaller than the maximum range that would be possible if all of the Peltier cells in a stack had the same temperature differential.
The need for dry well calibrators to operate over wide temperature ranges frequently requires that the Peltier cells used in the calibrators be operated at or near their maximum specified temperature differential. Unfortunately, operation of the Peltier cells at or near their maximum specified temperature differentials can severely limit the useful life of the cells. Frequent replacement of the Peltier cells can be very expensive, not only because of the cost of the cells, but also because of the cost of labor required to disassemble dry well calibrators to replace the cells and the downtime cost during such replacement. As a result, there is an inevitable tradeoff between achieving a wide operating range for dry well calibrators and achieving reliable performance.
There is therefore a need for a dry well calibrator using Peltier cells that can operate over a wide range of temperatures without unduly limiting the useful life of the Peltier cells.